Integrated circuits are made possible by processes which produce intricately patterned layers of materials on substrate surfaces. Producing patterned material on a substrate requires controlled methods for removal of exposed materials. Chemical etching is used for a variety of purposes including transferring a pattern in photoresist into underlying layers, thinning layers, or increasing lateral dimensions of features already present on the surface. Often it is desirable to have an etch process which etches one material faster than another. Such an etch process is said to be selective to the first material. As a result of the diversity of materials, circuits, and processes, etch processes have been developed with a selectivity towards a variety of materials.
Plasma deposition and etching processes for fabricating semiconductor integrated circuits have been in wide use for decades. These processes typically involve the formation of a plasma from gases that are exposed to electric fields of sufficient power inside the processing chamber to cause the gases to ionize. The temperatures needed to form these plasmas can be much lower than needed to thermally ionize the same gases. Thus, plasma generation processes can be used to generate reactive radical and ion species at significantly lower chamber processing temperatures than is possible by simply heating the gases. This allows the plasma to deposit and/or etch materials from substrate surfaces without raising the substrate temperature above a threshold that will melt, decompose, or otherwise damage materials on the substrate.
Exemplary plasma deposition processes include plasma-enhanced chemical vapor deposition (PECVD) of dielectric materials such as silicon oxide on exposed surfaces of a substrate wafer. Conventional PECVD involves the mixing of gases and/or deposition precursors in the processing chamber and striking a plasma from the gases to generate reactive species that react and deposit material on the substrate. The plasma is typically positioned close to the exposed surface of the substrate to facilitate the efficient deposition of the reaction products.
Similarly, plasma etching processes include exposing selected parts of the substrate to plasma activated etching species that chemically react and/or physically sputter materials from the substrate. The removal rates, selectivity, and direction of the plasma etched materials can be controlled with adjustments to the etchant gases, plasma excitation energy, and electrical bias between the substrate and charged plasma species, among other parameters. Some plasma techniques, such as high-density plasma chemical vapor deposition (HDP-CVD), rely on simultaneous plasma etching and deposition to deposit films on the substrate.
While plasma environments are generally less destructive to substrates than high-temperature deposition environments, they still create fabrication challenges. Etching precision can be a problem with energetic plasmas that over-etch shallow trenches and gaps. Energetic species in the plasmas, especially ionized species, can create unwanted reactions in a deposited material that adversely affect the material's performance. Thus, there is a need for systems and methods to provide more precise control over the plasma components that make contact with a substrate wafer during fabrication.